Ultrasound contrast agents (UCA) are small gas-filled bubbles with a stabilizing shell made from a variety of materials, such as polymer, protein or lipid. Other than the traditional applications of these agents in diagnostic ultrasound imaging, UCA have found relevance in therapeutic applications including targeted gene and drug delivery. These adaptable particles are currently being explored as protective therapeutic carriers and as cavitation nuclei to enhance delivery of their payload by sonoporation. Together these functions improve payload circulation half-life and release profiles as well as tissue selectivity and cell uptake. Regardless of the mode of action, it is advantageous, particularly in cancer therapy, for the bubbles to extravasate from the vasculature and arrive at the cellular target site for the desired effect.
Commercial UCA available today are typically designed to serve only as blood pool agents with diameters of 1-8 μm. Although previous methodologies have been developed to reduce bubble size, most of these strategies involve manipulations of microbubbles post formation, such as gradient separation by gravitational forces or by physical filtration or floatation. While effective for selecting nanosized bubbles, these methods introduce potential for sample contamination, reduce bubble yield and stability, and waste stock materials in addition to being labor intensive. Additionally, the applicability of microbubbles as carriers (e.g., in cancer therapy) has been limited by a large size, which typically confines them to the vasculature.